Mr HwjIl Miller on Jlln'r-Terracing. 271 



Somme Valley are ancient flood-margins of the river while 

 flowing in its present channel. He holds that in England 

 the valleys are adapted to hold rivers many times as great as 

 now, and supposes a " rain period," during which rivers, such 

 as the Aire in Yorkshire, had 125 times their present volume.* 



The same view has since been taken of terraces in the 

 Earn and Teith by the Rev. Thomas Brown.f Dana, too, 

 discarding his earlier views of the Connecticut terraces, now 

 traces them almost wholly to the effects of gigantic floods 

 caused by the melting glaciers of the great Ice Age in 

 America. The terraces mark stages in the rising or retreat- 

 ing deluge, which must, if so, have risen from 200 to 250 

 feet above modern low water. Dana is well aware, how- 

 ever, that floods of so great a magnitude would sweep away 

 all but the coarsest possible debris; and he calls in the 

 unequal elevation of the continent since glacial times to 

 lessen the gradient. J In this country there is no river on 

 which any inequality of elevation of which the raised 

 beaches give evidence could have any such effect ; and the 

 structure of the river-terraces is incompatible with this 

 almost cataclysmal explanation. 



Smaller climatic changes are less open to objection. 

 Whitney § believes that " gradual and uninterrupted action 

 of eroding forces " is unfavourable to the formation of river 

 terraces, and supposes a succession of "periods of drainage 

 alternating with periods of repose." Drew, in the Himalayan 

 Valleys, |1 seems to connect the river terraces wath variations 

 in the supply of debris, disengaged by glaciers among the 

 mountains. 



In Norway, Professor Kjerulf, like Hitchcock, has recog- 

 nised two classes of terraces : those in " closed situations," 

 or in basins hemmed in at the low^er end by barriers of 

 moraine or rock ; and those with an " open situation," where 



* On Quaternary Gravels (Quart. Jour. Geol. Soe., vol. xxv., p. 57). 

 t Trans. Roy. Soc, Edinb., vol. xxvi., 1870. 

 X See footnote, p. 269. 



§ J. D. Wliitney, Geol. of ^Yisconsin, vol. i., p. 3 08. 

 II Drew, Alluvial Deposits of tlie Upper Indus Basin (Quart. Jour. Geol. 

 Soc, vol. xxix., p. 441). 



